Research On Gamma-event Pulse Shape And Performance Optimization Of Spectrometry In Low-background BEGe

BEGe detector is a kind of high purity germanium detector which has a very small inner electrode and the consequent feature of single-carrier response,thus it has advantage in pulse shape discrimination.BEGe detector is mainly applied in gamma spectrometry and neutrinoless double beta decay which both need a low background level.As to a lowbackground BEGe facility based on good shielding conditions and material selections,pulse shape discrimination is an effective choice to further suppress the background level.For the dominant applications of BEGe detector,signal and background events are mostly induced by gamma events.In this dissertation,the physical mechanism and pulse behavior of gamma events in low-background BEGe detector are analyzed,and the application of pulse shape discrimination methods to the performance optimization of gamma spectrometry is studied.Base on the characterization of BEGe detector,the digital pulse acquisition system and software research platform for experiments and simulations related to pulse shape discrimination respectively are established in the beginning.In the software research platform,the simulation module can be used to study the physical mechanism of events in germanium crystals and evaluate the application performance of pulse shape discrimination methods,while the digital pulse processing module is the direct tool to realize pulse shape discrimination.The features of BEGe detector which relate to pulse shape discrimination are studied by simulation module.It turns out that the position sensitivity of BEGe detector is limited,which means it is hard to fully extract the interacting information of events;and electrons below 2.4 MeV deposit energy in the range of 1-2 mm in germanium crystals,which means it is hard to effectively discriminate single-electron and double-electron energy deposition events.The physical mechanism of gamma events in BEGe detector for gamma spectrometry is also studied by simulation module.It turns out that the distributions of numbers and positions of interaction points are different between fullenergy-peak signal events and compton-continuum background events,and this difference can serve as the theoretical foundation for the development of pulse shape discrimination methods which are dedicated to the suppression of compton-continuum background.Based on the analysis of the physical mechanism of gamma events,A/E method and rise-time ratio method are established in order to realize single-site event/multi-site event discrimination and event interaction-position discrimination respectively,and these two methods are expected to suppress the compton-continuum background in gamma spectrometry.The MDA optimization effects of the two methods in a typical lowbackground measurement condition are evaluated by simulation.The features of the two methods are deeply studied by experiments with point sources.The MDA corresponding to the 661 keV gamma ray can be improved to ~81 % of the former value by the two methods in the low-background measurement of a typical squid-ash sample,namely the measurement time can be reduced to ~65 % of the former value when achieving the same MDA level as before.The research of this dissertation sets a good example for the study of pulse shape discrimination methods in BEGe detector,and the two pulse shape discrimination methods established in this work have positive effect on the performance optimization of gamma spectrometry.